Printed circuit board with embedded thermocouple junctions

ABSTRACT

A first strip and a second strip of metal are embedded on a first layer of a printed circuit board (PCB) to form a junction. The junction has a first strip tail and a second strip tail. The first strip and the second strip are made of different metals. A first trace and a second trace are fabricated on a second layer to extend the junction to a first trace pad and a second trace pad, respectively. The first and second traces are fabricated on a second layer. The first and second traces have a first end one and a second end one, respectively. The first end one is connected to the first strip using a first via and the second end one is connected to the second strip using a second via. A first wire is inserted to the first via and a second wire to the second via to measure the temperature of the PCB.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This invention relates to printed circuit board (PCB) design. Inparticular, the invention relates to thermocouple junctions embedded inthe printed circuit board.

[0003] 2. Description of Related Art

[0004] Thermal dissipation increases as the frequency increases sincethermal dissipation is a function of the square of the frequency. Giventhat bus frequencies have been increasing in tandem with the processorfrequencies (e.g., speeds) and with the processor frequencies doublingevery 18 months, thermal management poses significant technologicalchallenges. In order to control the temperature of the criticalcomponents mounted on the PCB, it is necessary to monitor theirtemperatures accurately at or near the areas in which excessive heat isgenerated to prevent failure of these components. The current state ofthe art technique for measuring the temperature utilizes externallymounted thermocouple (TC) wire junction physically attached onto orbrought in contact with the surfaces of the PCB or components ofinterest. The other ends of these wires are typically soldered to padsor vias in the PCB. Since it is difficult or impossible to insert thethermocouple junctions directly into the area underneath the mountedcomponents, therefore, it is not possible to measure the temperaturedirectly and accurately using this technique. It is also important tomonitor and control the temperature of the PCB during a reflow processto ensure that the components are assembled under appropriate reflowconditions.

[0005] Therefore, there is need for better structure and technique toprovide a better measurement of the temperature of the components on PCBand the PCB itself.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The features and advantages of the present invention will becomeapparent from the following detailed description of the presentinvention in which:

[0007]FIG. 1 is a diagram illustrating a system including a conventionalthermocouple wire junction externally attached on top of a componentmounted on a printed circuit board (PCB).

[0008] FIGS. 2A-2F are diagrams illustrating top view layouts of metalstrips that formed a thermocouple junction according to one embodimentof the invention.

[0009]FIG. 3 is a diagram illustrating the TC junction embedded in thePCB according to one embodiment of the present invention.

[0010]FIG. 4 is a diagram illustrating an assembly having a substrateaccording to one embodiment of the invention.

DETAILED DESCRIPTION

[0011] In the following description, for purposes of explanation,numerous details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that these specific details are not required inorder to practice the present invention.

[0012]FIG. 1 is a diagram illustrating a system 100 including aconventional thermocouple wire junction externally attached on top of acomponent mounted on a printed circuit board (PCB).

[0013] The system 100 includes, among other things, a printed circuitboard (PCB) 110 having a top or surface layer 103, a component 106, athermocouple wire junction 105, thermocouple wires 101 and 102, and pads104 and 106.

[0014] The printed circuit board 110 is used for interconnectingintegrated circuit (IC) chips and other electronic components anddevices. The PCB 110 is formed by substrate that supports a plurality ofinsulated conductive trace layers. The insulated conductive trace layerstypically include surface conductive trace layers (i.e., top layer 103)and embedded trace layers with selected trace layers connected as aground plane and a power plane. Electronic components (i.e., component106), integrated circuits, and devices (not shown) are mounted on thesurface layer 103 by plated-through holes called vias (not shown). A viais a conducting hole between the multiple layers in the PCB 110. The viamay be made by drilling a via hole through the multilayer and platingthe via hole with a conductive material.

[0015] The component 106 is mounted onto the surface layer 103 of thePCB 110. The component 106 dissipates heat when it is powered. Thetemperature of the component is measured via the thermocouple wirejunction 105 attached to the surface of the component 106. Thethermocouple wire junction 105 is formed by spot welding one end of thethermocouple wire 101 to one end of the thermocouple wire 102 (asshown). It is necessary that the thermocouple wires 101 and 102 be madeof different materials in order to form the thermocouple junction 105.The other two ends of the thermocouple wires 101 and 102 are soldered topads/vias 104 and 106, respectively, or directly to a multimeter orvoltage sensing device. The temperature of the component 106, therefore,can be measured from the two pads/vias 104 and 106 or attaching directlyto an electronic device. It is contemplated that more components andtheir corresponding TC wire junctions and TC wires may be mounted ontothe PCB 110.

[0016] FIGS. 2A-2F are diagrams illustrating top view layouts of metalstrips formed in metal foils or layers from which a thermocouplejunction according to one embodiment of the invention is fabricated.

[0017]FIG. 2A illustrates a top view of a metal layer 201 having a metalstrip 202 laid on top of a PCB layer 212 (i.e., a preimpregnated orprepeg PCB laminate). The metal layer 201 is a metal foil or layerhaving a thickness “d” (not shown). The foil may be copper foil,constantan foil, or any kind of metallic foil or layer amenable toforming a viable junction with another foil. In one embodiment, themetal layer 201 is made of copper foil having the thickness “d” ofapproximately 5 mills. The metal strip 202 is formed at a desiredlocation by incorporating a set of two adjacent rectangular cutouts 203and 204. The cutouts 203 and 204 are formed by removing the coppermaterial by laser beam cutting or other standard material removaltechnique. The metal strip 202 has a width “w” of approximatelycomparable to the trace widths in the PCB. As shown in FIG. 2A, themetal strip 202 is formed in the middle of the set of cutouts 203 and204. The set of cutouts 203 and 204 is to be used later to form adistinct thermocouple junction (will be discussed in FIG. 2E).Additional strips can be formed by making similar cutouts at differentdesired locations. It is contemplated that other means such asphotochemical or dry etching techniques may be used in the process ofremoving the copper material. The cutouts and the metal strips may haveany shapes and sizes and at any desired locations in relation to themetal layer 201. Also, the thickness “d” and the width “w” may varydepending on the design specification.

[0018]FIG. 2B illustrates a top view of a metal layer 205 having a metalstrip 206. The metal layer 205 is made of different material from themetal layer 201. For example, if the metal layer 201 is made of copperthen the metal layer is made of material other than copper (i.e.,constantan). As stated before, in order to form the thermocouplejunction, it is necessary that the metal strips 206 and 202 be made ofdifferent materials. The process of making the metal strip 206 issimilar to the process of making the metal strip 202. The metal strip206 is positioned in a way that when the metal layers 205 and 201 areintegrated together, a thermocouple junction is formed using the metalstrips 202 and 206 (as shown in FIG. 2C).

[0019]FIG. 2C illustrates a top view of the metal layers 201 and 205attaching together. The metal layer 201 is overlaid on the metal layer205. In one embodiment, the overlaying strips 202 and 206 cross eachother. The two strips are spot welded to form a TC junction 207 at theirintersection. It is contemplated that the overlaying strips 202 and 206may be of different shapes and or sizes and may be in contact at adifferent position as long as when they are spot welded the TC junction207 having two tails made of different materials is formed.

[0020]FIG. 2D illustrates a spot weld of two different material layersor foils (i.e., copper-constantan foils). By using conventional PCBmanufacturing techniques, the laminate layer 212 is combined with theother laminate layers to form a multilayer PCB composite. In oneembodiment, the excess copper-constantan material is removed before theprocess of combining the laminate layer 212 with other layers. Thetechnique used to remove the excess material may be laser beam cutting,dry etching or wet etching (i.e., etching by chemical means). Wetetching, however, is not recommended given that the residual chemicalsat the junctions of the thermocouples could pose a long-term corrosionand reliability issues. The TC junction 207 and TC tails 221 and 222 areformed after the removal of the excess materials is shown in FIG. 2E.This structure has two tails 221 and 222 that are made of copper andconstantan, respectively. The TC tails 221 and 222 represent part of thestrips 202 and 206, respectively.

[0021]FIG. 2F illustrates a top view of a single PCB laminate layer 215on which two metal traces 216 and 217 are etched. In one embodiment, themetal traces 216 and 217 are made of copper material. The copper traces216 and 217 are fabricated to establish electrical contact between theTC junction 207 and the chip or module (not shown) measuring the voltageinduced at the junction. The copper traces 216 and 217 are connected topad 218 and 219 (as shown) and the other ends connect to via holes 220and 224. These via holes extend from the layer 215 to the other layer(i.e., layer 214) in the PCB. The via holes 220 and 224 are coated witha conductive material such as copper to establish contact to otherlayers in the PCB.

[0022] Since the copper or constantan traces 216 and 217 and TC junction207 are located on respective laminate layers 214 and 215, vias 220 and224 are needed to establish electrical contact between the two layers.Vias 220 and 224 are drilled into the PCB at the ends of the traces 216and 217 (at locations shown in FIG. 2F). These vias 220 and 224 arepositioned such that drilling the vias intersects the metal traces 216and 217 to the tails 221 and 222 of the TC junction 207. The point ofcontact of the vias 220 and 224 can be anywhere on the tails 221 and222, respectively. In one embodiment, the point of contact is at theends of the tails 221 and 222.

[0023]FIG. 3 is a diagram illustrating the TC junction 207 and TC tails221 and 222 embedded in the PCB according to one embodiment of thepresent invention.

[0024] The traces 216 and 217 are on the top layer 215 of the PCB. Thetraces 216 and 217 run from the vias 220 and 224 to the pads 218 and219, respectively, on the top layer 215. The TC junction 207 and itstails 221 and 222 are shown (hatched), to indicate its location, whichis buried underneath the surface layer 215. The vias 220 and 224 connectthe traces 216 and 217 on the top layer 215 to the tails 221 and 222embedded on layer 212. Voltages are measured at the pads 219 and 218.From voltage measurement, the temperature of the component on thesurface layer 215 can be determined.

[0025] In another embodiment, the TC junction 207 may be used inmonitoring and controlling the temperature of the PCB itself during areflow process in which various components are surface mount assembledonto the PCB. It is important to monitor and control the temperatureduring the reflow process because it ensures that these components areassembled under appropriate and uniform reflow conditions. Currently,this is accomplished through positioning thermocouple wire junctiondirectly onto the PCB surfaces prior to the reflow process. The embeddedthermocouple junction 207 embedded in layer 214 allows monitoring thePCB temperature at the vias 220 and 224 of the top layer 215 withoutexternally attaching thermocouple wires onto the PCB and circumventingthe time-consuming manual work involved in attaching the TC wires.

[0026]FIG. 4 is a diagram illustrating an assembly having a substrateaccording to one embodiment of the invention. The assembly 400 includesa printed circuit board (PCB) 412. This assembly 400 also includescomponents and devices (not shown) such as a processor die 416. The PCB412 includes multiple layers for interconnecting integrated circuit (IC)chips and other electronic component and devices. The multiple layeredPCB 412 is formed from the substrate supporting a plurality of insultedconductive trace layers. The layers typically include a surfaceconductive trace layer (i.e., surface layer 215) and embedded tracelayers (i.e., layer 212). Integrated circuits (i.e., processor die 416)and electronic components and devices are mounted on an outer (i.e.,surface) of the multiple layer PCB 412 and selectively connected to thetrace layers by plated-through holes called vias.

[0027] The processor die 416 is an integrated circuit package. To havean accurate measurement of the heat dissipated from the processor die416, a thermocouple junction (i.e., thermocouple junction 207) isembedded in layer 414 (i.e., layer 214 in FIG. 2E) one of the multiplelayers in the PCB 412. In one embodiment, the thermocouple junction isembedded in the layer adjacent to the surface layer 420. Thethermocouple junction is embedded underneath or nearby the processor dieor integrated circuit 416 to receive a more direct and accuratetemperature reading. The reading is taken at the pads/vias (i.e.,pads/vias 218 and 219) on the surface layer 420.

[0028] While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments of the invention, which areapparent to persons skilled in the art to which the invention pertainsare deemed to lie within the spirit and scope of the invention.

What is claimed is:
 1. A method comprising: embedding a first strip anda second strip of metal on a first layer of a printed circuit board(PCB) to form a junction having a first strip tail and a second striptail, the first and second strips of metal being made of differentmetals; extending the junction to a first trace pad and a second tracepad using a first trace and a second trace, the first and second tracesbeing fabricated on a second layer, the first trace having a first endone and the second trace having a second end one, the first end onebeing connected to the first strip tail using a first via and the secondend one being connected to the second strip tail using a second via; andmeasuring a temperature of the PCB from the first and second vias. 2.The method of claim 1 wherein the first and second strips are aligned toform the junction at the desired location in the PCB.
 3. The method ofclaim 2 wherein embedding the first and second strips of metalcomprises: forming the first strip on a first metal layer; forming thesecond strip on a second metal layer; spot welding the first strip tothe second strip to form a junction at a desired location.
 4. The methodof claim 3 further comprising: assembling a component onto the PCB. 5.The method of claim 4 wherein the desired location is located within aclose proximity of the component, underneath the component and withinthe PCB.
 6. The method of claim 5 furthers comprising measuring avoltage potential difference across the junction at the first and secondtrace pads.
 7. The method of claim 1 wherein the junction is athermocouple junction.
 8. The method of claim 1 wherein the first layeris a laminate layer.
 9. The method of claim 2 wherein the first metallayer is made of copper material.
 10. The method of claim 2 wherein thesecond metal layer is made of constantan material.
 11. The method ofclaim 1 wherein the first and second via holes is coated with aconductive material.
 12. An apparatus comprising: a first strip and asecond strip of metal embedded on a first layer of a printed circuitboard (PCB) to form a junction having a first strip tail and a secondstrip tail, the first and second strips of metal being made of differentmetals; and a first trace pad and a second trace pad extended thejunction using a first trace and a second trace, the first and secondtraces being fabricated on a second layer, the first trace having afirst end one and the second trace having a second end one, the firstend one being connected to the first strip tail using a first via andthe second end one being connected to the second strip tail using asecond via, the first and second vias being used to measure atemperature of the PCB.
 13. The apparatus of claim 12 wherein the firstand second strips are aligned to form the junction at a desired locationin the PCB.
 14. The apparatus of claim 13 wherein the embedded first andsecond strips of metal are spot welded to form a junction.
 15. Theapparatus of claim 12 wherein the junction is a thermocouple junction.16. The apparatus of claim 12 wherein the first layer is a laminatelayer.
 17. The apparatus of claim 13 wherein the first metal layer ismade of copper material.
 18. The apparatus of claim 13 wherein thesecond metal layer is made of constantan material.
 19. An assemblycomprising: a printed circuit board (PCB) having multiple layers; adevice mounted onto the PCB; and a thermocouple junction embedded in oneof the multiple layers to provide accurate temperature measurement ofheat dissipated from the device, the thermocouple junction comprising: afirst strip and a second strip of metal embedded on a first layer of thePCB to form a junction having a first strip tail and a second striptail, the first and second strips of metal being made of differentmetals, a first trace pad and a second trace pad extended the junctionusing a first trace and a second trace, the first and second tracesbeing fabricated on a second layer, the first trace having a first endone and the second trace having a second end one, the first end onebeing connected to the first strip tail using a first via and the secondend one being connected to the second strip tail using a second via, anda first wire inserted to the first via and a second wire inserted to thesecond via to measure a temperature of the PCB.
 20. The assembly ofclaim 19 wherein the embedded first and second strips of metal is spotwelded to form a junction at a desired location
 21. The assembly ofclaim 20 wherein the first and second strips are aligned to form thejunction at the desired location in the PCB.
 22. The assembly of claim19 wherein the junction is a thermocouple junction.
 23. The assembly ofclaim 19 wherein the first layer is a laminate layer.
 24. The assemblyof claim 20 wherein the first metal layer is made of copper material.25. The assembly of claim 20 wherein the second metal layer is made ofconstantan material.